I was going mining and realized I forgot to grab a pickaxe so I quickly switched to creative to put one in my inventory. When I did, I had two empty slots in the bottom screen bar, and so it automatically put two pick axes in the slots. When I switched back to survival, I found that while I was mining the durability would go down for both at the exact same time even though I was only using one.
After this, I threw the pickaxe and when I picked it up again I had only one.

Wouldn't be surprised if someone has already reported this.